Fully-controlled pumped storage static start frequency converter and control method thereof

Abstract

The invention relates to the field of motors and power electronics, and provides a full-control type pumped storage static starting frequency converter and a control method thereof.The method comprises the steps that firstly, in the starting process of the static starting frequency converter, actual starting time and planned starting time are compared, and when the actual starting time is larger than the planned starting time, the frequency converter is started; calculating an adjusting current based on a time difference between the two, adjusting the rectification unit according to the adjusting current and a sampling current at an alternating current side of the rectification unit, and increasing a motor torque so that the motor is started within a planned time; the planned starting time is the time used when the rotating speed of the motor reaches a set threshold value from zero in the successful starting process according to historical data, and the actual starting time refers to the time when the rotating speed of the motor actually reaches the set threshold value; and then, after the motor is started, the output voltage of the inverter unit is adjusted according to the grid-connected voltage requirement, so that the output voltage of the motor reaches the grid-connected condition.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electric machines and power electronics, in particular to a full-controlled pumped storage static starting frequency converter and a control method thereof. BACKGROUND

[0002] As a key facility in the power system, pumped storage power station undertakes multiple functions such as peak load shifting, frequency and phase modulation, emergency backup, etc., which is crucial to the stable operation of the power grid. With the rapid development of new energy, the role of pumped storage power station in the power system becomes more and more important. As a core component of pumped storage power station, the static starting frequency converter converts the power frequency alternating current into a constant direct current power through the rectifier circuit, and then converts the direct current into an alternating current with variable frequency through the inverter to control the rotation of the motor.

[0003] The starting process of the static starting frequency converter is divided into two stages of starting acceleration and synchronization adjustment. In the traditional static starting frequency converter, the rectifier circuit and the inverter are composed of ordinary thyristors. In the initial stage of starting, the motor speed is small, and the back electromotive force provided cannot control the thyristor to open and close as required. Therefore, the starting process needs to be divided into three stages of discontinuous commutation operation, discontinuous-natural commutation operation and natural commutation operation. It generally takes about 4 minutes from the start of static starting to synchronization and grid connection. The full-controlled static starting frequency converter uses IGBT / IEGT to compose the rectifier circuit and the inverter, which eliminates the fixed ring phase logic of the traditional static starting frequency converter and improves the controllability and stability of the frequency converter.

[0004] In the starting of the traditional static starting frequency converter, the motor speed curve needs to be preset in advance, and the deviation between the actual motor speed and the speed curve is judged in real time, so as to adjust the motor speed in time. This starting method can stably control the motor speed to change according to the predetermined curve, but in the adjustment process, only the preset motor speed curve is used as the adjustment basis, without considering the actual working condition. When the load fluctuates or the power grid is disturbed, the motor cannot be adjusted in time according to the actual working condition, which easily leads to starting failure, poor flexibility of the overall adjustment process and long time required. SUMMARY

[0005] The purpose of the present application is to provide a full-controlled pumped storage static starting frequency converter and a control method thereof, to solve the problem that the existing static starting frequency converter control method cannot adjust the motor in time according to the actual working condition.

[0006] The present application provides a full-controlled pumped storage static starting frequency converter control method, which comprises the following steps: 1) In the process of static start frequency converter starting, the actual starting time of the motor is compared with the planned starting time, when the actual starting time is greater than the planned starting time, the adjustment current is calculated based on the time difference between the two, and the rectifier unit is controlled according to the adjustment current and the sampling current of the AC side of the rectifier unit of the static start frequency converter, so as to increase the motor torque, so that the motor completes the starting in the planned time; The planned starting time is the time used by the motor speed from zero to the set threshold value in the successful starting process according to the historical data, and the actual starting time refers to the time when the motor speed actually reaches the set threshold value; 2) After the motor completes the starting, the output of the inverter unit of the static start frequency converter is controlled according to the grid-connected voltage demand, so as to adjust the motor speed and the excitation current, and make the motor output reach the grid-connected condition.

[0007] Further, the calculation method of the modulation current is: the difference between the actual starting time and the planned starting time is proportional integral processing, the motor torque that needs to be additionally increased is calculated, and the motor stator current corresponding to the motor torque is calculated as the adjustment current according to the proportional relationship between the motor torque and the motor stator current.

[0008] Further, the process of adjusting the output voltage of the inverter unit according to the grid-connected voltage demand includes: obtaining the voltage given value according to the grid-connected voltage demand, calculating the modulation signal given signal by using the difference between the voltage given value and the output voltage of the inverter unit, controlling the output voltage of the inverter unit according to the modulation signal given signal, and controlling the motor excitation current and the motor speed based on the output voltage of the inverter unit.

[0009] Further, the strategy of regulating the output voltage signal of the inverter unit includes the control strategy taking the output voltage of the inverter unit as the control target, the control strategy taking the output current of the inverter unit as the control target, and the pulse width modulation control.

[0010] The full-control type pumped storage static starting frequency converter comprises a rectifying unit and an inverting unit, and further comprises a control unit, a voltage collecting unit and a current collecting unit, the voltage collecting unit is used to acquire the output voltage of the inverting unit, the current collecting unit is used to acquire the sampling current on the AC side of the rectifying unit, the control unit comprises a rotating speed regulator, a current regulator and an inverting regulating unit, the rotating speed regulator is used to calculate the regulating current based on the time difference between the actual starting time of the motor and the planned starting time, the current regulator is used to control the rectifying unit according to the regulating current and the sampling current on the AC side of the rectifying unit acquired by the current collecting unit, so as to increase the motor torque and make the motor complete the starting within the planned time, the planned starting time is the time used by the motor rotating speed to increase from zero to a set threshold value in the successful starting process according to the historical data, and the actual starting time refers to the time when the motor rotating speed actually reaches the set threshold value, and the inverting regulating unit is used to regulate the motor rotating speed and the excitation current according to the grid-connected voltage demand after the motor completes the starting, so as to make the motor output reach the grid-connected condition.

[0011] Further, the rotating speed regulator proportionally and integrally processes the difference between the actual starting time and the planned starting time, calculates the motor torque that needs to be additionally increased, and calculates the motor stator current corresponding to the motor torque as the regulating current according to the proportional relationship between the motor torque and the motor stator current.

[0012] Further, the process that the inverting regulating unit regulates the output voltage of the inverting unit according to the grid-connected voltage demand comprises that the inverting regulating unit acquires a voltage given value according to the grid-connected voltage demand, calculates a modulation signal given signal by using the difference between the voltage given value and the output voltage of the inverting unit, controls the output voltage of the inverting unit according to the modulation signal given signal, and controls the motor excitation current and the motor rotating speed based on the output voltage of the inverting unit.

[0013] Further, the strategy for regulating the output voltage signal of the inverting unit comprises a control strategy taking the output voltage of the inverting unit as the control target, a control strategy taking the output current of the inverting unit as the control target and a pulse width modulation control.

[0014] Further, the power switching devices in the rectifying unit and the inverting unit are IGBTs or IEGTs.

[0015] The beneficial effects of the present application are: as an improved invention, the present application detects the motor speed in real time during the starting process of the static starting frequency converter, takes the time when the motor speed reaches the set threshold value as the actual starting time, compares the actual starting time reflecting the actual working condition with the pre-set planned starting time, calculates the adjustment current according to the difference between the actual starting time and the planned starting time when the actual starting time is greater than the planned starting time, controls the rectifier unit of the frequency converter by using the adjustment current and the sampling current on the alternating current side of the rectifier to control the motor to increase the torque, so that the motor reaches the rated speed within the planned time, and the starting of the motor is completed. In the whole control process, the present application can compensate and control according to the actual starting time of the motor speed, so that the starting of the motor can be completed within the planned time, and the adjustment of the motor according to the pre-set motor speed curve is avoided, and the static starting frequency converter is controlled according to the actual working condition to adjust the motor speed in time. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a method flow diagram; Figure 2 is a main circuit topology of the full-controlled pumped storage static starting frequency converter; Figure 3 is a current control method of the full-controlled pumped storage static starting frequency converter; Figure 4 is the frequency and speed curve of the energy storage unit in the motor starting process in simulation; Figure 5 is the overall three-phase voltage waveform in the motor starting process in simulation; Figure 6 is the local three-phase voltage waveform in the motor starting process in simulation; Figure 7 is the three-phase voltage waveform line in the motor starting process in simulation; Figure 8 is the DC bus current in the motor starting process in experiment; Figure 9 is the torque in the motor starting process in experiment; Figure 10 is the frequency lifting curve in the motor starting process in experiment; Figure 11 is the three-phase voltage waveform in the motor starting process in experiment. DETAILED DESCRIPTION

[0017] The specific embodiments of the present application will be further described in combination with the drawings.

[0018] The application compares the actual starting time with the planned starting time during the starting process of the static starting frequency converter, and when the actual starting time is greater than the planned starting time, the speed of the static starting frequency converter is adjusted by calculating the difference between the two, so that the motor reaches the rated speed within the planned time.

[0019] Full-control pumped storage static starting frequency converter control method embodiment As shown in Figure 2 Full-control pumped storage static starting frequency converter mainly includes input reactor, rectifier unit, inverter unit, control unit and excitation unit. The AC side of the rectifier unit is connected to the power grid through the input reactor, and a transformer is generally provided between the input reactor and the power grid to avoid damage to the rectifier unit by high voltage. The DC side of the rectifier unit is connected to the input end of the inverter unit, and a DC reactor is generally provided between the rectifier unit and the inverter unit to ensure that the current flowing into the input end of the inverter unit is DC current, and the output end of the inverter unit is connected to the motor and the input of the excitation unit. The control unit is used to control the frequency converter to adjust the motor speed according to the current motor speed, so that the motor can start within the planned time, and the control unit includes a speed regulator and a current regulator.

[0020] As shown in Figure 3 The control unit includes a speed regulator, a current regulator and an inverter adjustment unit, the speed regulator is used to calculate the adjustment current according to the difference between the planned starting time and the actual starting time; the current regulator is usually used to control the rectifier unit according to the adjustment current and the current on the AC side of the rectifier unit, thereby controlling the starting process of the motor. The inverter adjustment unit is usually used to control the inverter output according to the modulation signal given, thereby further adjusting the motor speed and excitation current after the motor completes the start, so that the motor output reaches the grid-connected condition.

[0021] The starting principle of the full-control pumped storage static starting frequency converter is: the AC power of the power grid enters the rectifier unit through the transformer and the input reactor to be converted into DC power, and then flows into the inverter unit through the DC reactor to be converted into frequency-controllable AC power, and the AC power is input into the motor stator. At the same time, the output voltage of the inverter unit is input into the excitation unit after being processed by the voltage regulator, and the excitation regulator in the excitation unit inputs DC power to the motor rotor according to the output of the inverter unit, so that the motor starts to rotate and generates electricity and gradually meets the grid-connected requirements, realizing soft start of the entire pumped storage unit.

[0022] As shown in Figure 1 In order to realize efficient and stable starting of the motor and complete grid connection within the planned time, the application proposes a static starting frequency converter control method, which is divided into three stages: 1) Starting stage: start the motor through the static starting frequency converter.

[0023] The closed circuit breaker CB1 and the circuit breaker CB2 connect the power grid AC power to the static start frequency converter. The AC power passes through the transformer and the input reactor and enters the rectifier unit. The rectifier unit starts to work and enters the soft start running state. The AC power is converted to DC power by the rectifier unit. The DC power enters the inverter unit and is converted to AC power of a specific frequency to enter the motor stator. At the same time, the output of the inverter unit is processed by the voltage regulator and input to the excitation unit. The excitation regulator in the excitation unit outputs DC power to the motor rotor according to the processed output of the inverter unit, and the motor starts to rotate.

[0024] 2) Soft start running phase: control the motor to reach the rated speed at the planned start time.

[0025] When the motor starts to rotate, the motor speed is detected in real time by the speed sensor. When the motor speed reaches the set threshold, the actual start time is determined. The speed regulator compares the actual start time with the planned start time. When the actual start time is greater than the planned start time, the speed regulator calculates the adjustment current based on the time difference between the two. Specifically, when the actual start time is greater than the planned start time, the difference between the actual start time and the planned start time is calculated according to the proportional integral algorithm to obtain the additional motor torque required. The motor torque is proportional to the stator current, so the additional motor torque required can be converted to the corresponding adjustment current. When the actual start time is less than the planned start time, it indicates that the motor speed is too fast at this time, and in actual operation, the motor speed is usually not actively reduced to slow down the motor, but only records or alarms in this case. The current acquisition unit acquires the sampling current on the AC side of the rectifier unit. The current regulator controls the rectifier unit according to the adjustment current and the sampling current on the AC side of the rectifier unit to increase the motor torque, so that the motor completes the start in the planned time. Specifically, the current regulator compares the modulation current with the sampling current on the AC side of the rectifier unit to calculate the output instruction of the rectifier unit. The rectifier unit responds to the instruction to increase the output DC voltage and current. Further, as the rectifier unit responds to the instruction, the DC power in the circuit increases, the inverter unit converts the increased DC power into AC power of higher amplitude / frequency, and outputs the AC power to the motor stator, thereby increasing the electromagnetic torque of the motor and accelerating the motor start.

[0026] Wherein, the time when the motor speed actually reaches the set threshold value is taken as the actual starting time, the time when the motor speed reaches the set threshold value in the successful starting process in the historical data is taken as the planned starting time, and the planned starting time can be dynamically adjusted within a reasonable range according to different actual situations. When the motor is just started, the motor is in the stage of overcoming static friction, the speed measurement is unstable, the dynamic characteristics are complex and the nonlinearity is strong, and after the motor speed reaches 10% of the rated speed, the motor is out of the unstable region at low speed, and the relationship between the speed and the time has a relatively obvious linear relationship, therefore, as an embodiment, the set threshold value is 10% of the rated speed of the unit. According to the actual engineering requirements, the value range of the set threshold value is 5% of the rated speed ~ 15% of the rated speed, and the core target of selecting the set threshold value is to avoid the unstable region at the initial stage of starting.

[0027] 3) Synchronization adjustment stage: adjust the output voltage to meet the grid-connected condition.

[0028] When the motor speed reaches the rated speed, that is, the motor completes the starting, the control unit of the static starting frequency converter collects the output voltage signal of the inverter unit in real time through the voltage collection module, at the same time, the static starting frequency converter sets the voltage given value according to the grid-connected requirement of the power grid, calculates the modulation signal by using the difference between the output voltage of the inverter unit and the voltage given value, and controls the conduction or turn-off of the IGBT / IEGT in the inverter unit by using the modulation signal, to form a voltage closed loop and realize the control of the output voltage of the inverter unit. The output voltage signal at the output end of the inverter unit is transmitted to the excitation system after being processed by the voltage regulator, to control the excitation regulator in the excitation system to input the direct current (excitation current) to the motor rotor, so as to adjust the amplitude, frequency and phase of the motor output, to meet the synchronous grid-connected requirement. In order to ensure the stability of the output voltage of the inverter unit, the control strategy of the output voltage of the inverter unit can be selected from the voltage source inverter (VSI) control, the current source inverter (CSI) control or the pulse width modulation (PWM) control.

[0029] 4) Grid-connected stage.

[0030] When the motor output voltage meets the grid-connected condition, the grid-connected is completed by closing the synchronization circuit breaker CB3.

[0031] As shown in Figure 4 , when the control method of the static starting frequency converter is adopted, the motor is stably running at 200 revolutions per minute after starting for 150 seconds, and the frequency of the output voltage is stably at 50Hz, which indicates that the motor successfully completes the frequency conversion starting and enters the stable running mode. As shown in Figure 5 , Figure 6 and Figure 7The waveform can see that the method of the application exhibits smooth transient transition characteristics in the motor starting process, excellent harmonic suppression ability, specifically manifested as no violent oscillation in the overall waveform and continuous voltage without distortion, finally realizing the output voltage frequency accurate and stable at 50Hz, and the waveform line is strictly synchronous. The smooth switching effect from open-loop acceleration to closed-loop regulation is verified throughout, ensuring that the motor quickly reaches the required power frequency state for grid connection, providing reliable support for the safe closing of the synchronous circuit breaker and grid connection.

[0032] In order to further verify the reliability of the method proposed in the application, the variation characteristics of the motor side voltage during the motor starting process using the method proposed in the application are studied, and the experimental results are shown in Figures 8-11 . As can be seen from Figure 10 , during the motor starting stage, the voltage amplitude of the machine side slowly increases with the increase of speed and frequency, and from Figure 7 , the three-phase voltage waveform curve can be seen that the voltage phase difference is 120°. This phenomenon conforms to the basic principle of motor operation, and reflects the inherent relationship between voltage, speed and frequency. When the motor completes the frequency converter starting and enters the stable running state, the voltage amplitude of the machine side remains constant, which matches the stable running state of the speed and frequency, not only ensures the power supply quality of the motor during stable operation, but also provides stable voltage input for other equipment connected to the motor side, which is conducive to the stable operation of the whole system. From Figure 8 , Figure 9 and Figure 11 , the experimental data can be known that the method of the application exhibits small bus current fluctuation, stable dynamic response of output torque and low three-phase voltage waveform synchronization and harmonic distortion during the motor starting process, which comprehensively verifies the effectiveness of the control strategy for the whole process of motor starting, ensuring the safe and reliable operation from acceleration to grid connection stage.

[0033] Full-bridge static starting frequency converter for pumped storage power station The application provides a full-control pumped storage static starting frequency converter, which comprises a rectifying unit, an inverting unit, a control unit, a voltage collecting unit and a current collecting unit, the voltage collecting unit is used for acquiring the output voltage of the inverting unit, the current collecting unit is used for acquiring the sampling current on the alternating current side of the rectifying unit, the control unit comprises a rotating speed regulator, a current regulator and an inverting regulating unit, the rotating speed regulator is used for calculating the regulating current based on the time difference between the actual starting time of the motor and the planned starting time, the current regulator is used for controlling the rectifying unit according to the regulating current and the sampling current on the alternating current side of the rectifying unit acquired by the current collecting unit, so as to increase the motor torque and make the motor complete the starting within the planned time, the planned starting time is the time used for the motor rotating speed to increase from zero to a set threshold value in the successful starting process according to historical data, and the actual starting time refers to the time when the motor rotating speed actually reaches the set threshold value; the inverting regulating unit is used for regulating the motor rotating speed and the excitation current according to the grid-connected voltage demand after the motor completes the starting, so that the motor output reaches the grid-connected condition.

[0034] The planned starting time is the time used for the motor rotating speed to increase from zero to a set threshold value in the successful starting process according to historical data, and the actual starting time refers to the time when the motor rotating speed actually reaches the set threshold value; the inverting voltage regulating unit is used for regulating the output voltage of the inverting unit according to the grid-connected voltage demand after the motor completes the starting, so that the motor output voltage reaches the grid-connected condition.

[0035] The specific implementation process has been described in detail in the method embodiment, and will not be repeated here.

Claims

1. A fully controllable pumped storage static start frequency converter control method, characterized in that the steps are as follows: include: 1) During the startup process of the static start inverter, the actual startup time of the motor is compared with the planned startup time. When the actual startup time is greater than the planned startup time, the regulating current is calculated based on the time difference between the two. The rectifier unit is controlled according to the regulating current and the sampled current on the AC side of the rectifier unit of the static start inverter to increase the motor torque and enable the motor to start within the planned time. The planned startup time is based on the time taken for the motor speed to rise from zero to a set threshold during successful startup in historical data. The actual startup time refers to the time it takes for the motor speed to actually reach the set threshold. 2) After the motor has started, the output of the inverter unit of the static starter inverter is controlled according to the grid voltage requirements to adjust the motor speed and excitation current so that the motor output meets the grid connection conditions.

2. The fully controllable pumped storage static start frequency converter control method according to claim 1, characterized in that, The method for calculating the modulation current is as follows: the difference between the actual start time and the planned start time is processed by proportional integration to calculate the additional motor torque required. Based on the proportional relationship between the motor torque and the motor stator current, the motor stator current corresponding to the motor torque is calculated as the regulating current.

3. The fully controllable pumped storage static start frequency converter control method according to claim 1, characterized in that, The process of adjusting the output voltage of the inverter unit according to the grid-connected voltage demand includes: obtaining the voltage setpoint according to the grid-connected voltage demand; calculating the modulation signal setpoint using the voltage setpoint and the voltage difference at the output terminal of the inverter unit; controlling the output voltage of the inverter unit according to the modulation signal setpoint; and controlling the motor excitation current and motor speed based on the output voltage of the inverter unit.

4. The fully controllable pumped storage static start frequency converter control method according to claim 1, characterized in that, The strategies for regulating the output voltage signal of the inverter unit include control strategies with the output voltage of the inverter unit as the control target, control strategies with the output current of the inverter unit as the control target, and pulse width modulation control.

5. A fully controllable pumped storage static start frequency converter, comprising a rectifier unit and an inverter unit, characterized in that, It also includes a control unit, a voltage acquisition unit, and a current acquisition unit. The voltage acquisition unit is used to acquire the output voltage of the inverter unit, and the current acquisition unit is used to acquire the sampled current on the AC side of the rectifier unit. The control unit includes a speed regulator, a current regulator, and an inverter regulation unit. The speed regulator is used to calculate the regulating current based on the time difference between the actual motor start time and the planned start time. The current regulator is used to control the rectifier unit according to the regulating current and the sampled current on the AC side of the rectifier unit acquired by the current acquisition unit, so as to increase the motor torque and enable the motor to start within the planned time. The planned start time is based on the time taken for the motor speed to rise from zero to a set threshold during successful start-up in historical data. The actual start time refers to the time when the motor speed actually reaches the set threshold. The inverter regulation unit is used to adjust the motor speed and excitation current according to the grid voltage requirements after the motor has started, so that the motor output meets the grid connection conditions.

6. The fully controllable pumped storage static start frequency converter according to claim 5, characterized in that, The speed regulator performs proportional-integral processing on the difference between the actual start time and the planned start time to calculate the additional motor torque required. Based on the proportional relationship between motor torque and motor stator current, it calculates the motor stator current corresponding to the motor torque as the regulating current.

7. The fully controllable pumped storage static start frequency converter according to claim 5, characterized in that, The process of the inverter regulation unit adjusting the output voltage of the inverter unit according to the grid voltage demand includes: the inverter regulation unit obtains the voltage setpoint according to the grid voltage demand, calculates the modulation signal setpoint signal using the voltage setpoint and the voltage difference at the output terminal of the inverter unit, controls the output voltage of the inverter unit according to the modulation signal setpoint signal, and controls the motor excitation current and motor speed based on the output voltage of the inverter unit.

8. The fully controllable pumped storage static start frequency converter according to claim 5, characterized in that, The strategies for regulating the output voltage signal of the inverter unit include control strategies with the output voltage of the inverter unit as the control target, control strategies with the output current of the inverter unit as the control target, and pulse width modulation control.

9. The fully controllable pumped storage static start frequency converter according to claim 5, characterized in that, Both the rectifier unit and the inverter unit use IGBTs or IEGTs as power switching devices.

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